1. Field of the Invention
This invention relates generally to a concentration measuring system and more particularly to a system for measuring very low concentrations of high molecular weight polymers in solution.
2. Description of the Prior Art
In various situations, it has become important to be able to measure the concentration of high molecular weight polymers in solution. Where these concentrations are very low, such as 1-10 ppm, it has been particularly difficult to obtain quick and easy measurements. There have been no instruments or methods which are capable of quick, accurate, inexpensive and repeatable measurement of such very low concentrations. Furthermore, the devices which are available are sensitive to such variables as the quality of the solvent, with respect to its turbidity.
Methods which have been used to estimate or measure low concentrations of polymers in solution include nephelometric, colorimetric and complexation methods. The analysis which depends on spectrophotometric absorption measurements is subject to several different interferences. The solvent must be of high quality and free from suspended solids. Gravimetric and chromatographic methods suffer from low quality solvents. Further limitations on precipitation techniques include the time necessary and the expense of the reagents. Chromatographic analysis requires a higher concentration of polymer. Many of the methods are only applicable to particular polymers, and only in concentrations above 10 ppm.
One device developed by the United States Navy for measuring a polymer in solution is shown in U.S. Pat. No. 3,924,448. In order to measure a high concentration of polymer in solution, samples of the solution are diluted according to a specific ratio. Incoming water flows along a straight pipe containing a differential pressure gauge. The drop in pressure of the pure water over this length of pipe is measured. A small amount of solution is then added to the pure water to form the very low concentration solution. The solution is then measured by a differential pressure gauge in the same manner in which the pure water was measured above. Using the two pressure differentials, a percent drag reduction may be calculated. This low concentration can then be determined by a comparison with the calibration data relating concentration to percent drag reduction. After calculating the low concentration values, it is possible to compute the original higher concentration values using the dilution ratio figure. The pressure differentials measured in this system utilize a straight piece of pipe for measurement. The utility of this device is limited, however, since it is not possible to greatly vary the flow rate of the system. Further, since the pressure taps for both water and diluted solution are machined into the lines, the distance over which the pressure drop is measured is rigidly fixed. The variability of the system is therefore greatly restricted since neither the flow rate nor the pressure drop distance is adjustable.
Another system developed by the U.S. Navy is described in U.S. Pat. Nos. 3,327,522 and 3,420,096. In the first patent, a device for measuring the percent drag reduction by measuring a pressure differential along a straight piece of pipe is shown. The sample to be measured is driven along the pipe by a constant speed pump. The second patent uses measurements developed in the system disclosed in first patent in order to measure the molecular weight of polymers in solution. Calibration data is accumulated using known polymers and known concentrations. By measuring drag reduction for various concentrations, the polymer molecular weight may be established. The disadvantage of this system is the use of a constant speed pump which limits the range of flow rates available and the fixed distance between pressure tops. Also, the plunger-type sample chamber is not convenient for continuous, on-line processes or rapid repetitive sample measurements. Further, this system does not attempt to utilize the drag reduction information to measure concentrations of polymers.